1. Field of the Invention
The present invention relates to a voltage regulator for outputting a constant voltage, and more specifically, to an overcurrent protective circuit for protecting a circuit by reducing an output current when an overcurrent flows to an output terminal.
2. Description of the Related Art
A voltage regulator inputs a high power supply voltage from a power supply such as a lithium ion secondary cell and a battery, and outputs a voltage lower than the power supply voltage to a device such as a microcontroller. If an output terminal of the voltage regulator is short-circuited to a GND voltage (0 V) for some reason, the voltage regulator controls a resistance value of an output transistor to be decreased, and hence a large current flows to the output terminal. The voltage regulator needs an overcurrent protective circuit in order to protect the circuit because a large current flows to the output terminal.
FIG. 3 is a block diagram of a related-art voltage regulator. The related-art voltage regulator includes a constant voltage control circuit 320 and an overcurrent protective circuit 321.
An input voltage source 101 is connected between a power supply terminal 10 and a ground terminal 11. A load resistor 103 is connected between an output terminal 12 and the ground terminal 11. A resistor 111 and an output transistor 102 are connected in series between the power supply terminal 10 and the output terminal 12.
The constant voltage control circuit 320 is now described. A differential amplifier circuit 304 has an inverting input terminal to which a feedback voltage Vfb obtained by dividing an output voltage Vout with use of a voltage divider resistor circuit 306 is input, and has a non-inverting input terminal to which a reference voltage Vref of a reference voltage circuit 305 is input. The differential amplifier circuit 304 outputs a current I1 from its output terminal. An NPN transistor 308 and a resistor 307 form a common source amplifier circuit. The output transistor 102 has a gate to which an output voltage Vdry of the common source amplifier circuit is input, to thereby form a negative feedback for controlling the output voltage Vout so as to control the output voltage Vout to a set voltage.
The overcurrent protective circuit 321 is now described. A differential amplifier circuit 312 has a non-inverting input terminal to which a voltage Vin of the input voltage source 101 is input, and has an inverting input terminal to which a voltage VR generated in the resistor 111 is input. A differential amplifier circuit 313 has a non-inverting input terminal to which an output voltage of the differential amplifier circuit 312 is input, and has an inverting input terminal to which a reference voltage Vref2 of a reference voltage circuit 314 is input. The differential amplifier circuit 313 outputs a current I2. The current I2 is mirrored by an NPN transistor 315 and an NPN transistor 316 to be a current I3. The NPN transistor 316 has a collector connected to an output of the differential amplifier circuit 304.
In this case, when the output terminal 12 is short-circuited to the ground terminal 11, the feedback voltage Vfb approaches a ground voltage Vss, with the result that the output voltage Vdry of the common source amplifier circuit decreases. Then, the output transistor 102 is turned on to increase an output current Iout. As a result, the voltage VR decreases due to a voltage drop caused by the resistor 111, and then the output voltage of the differential amplifier circuit 312 increases. When the output voltage of the differential amplifier circuit 312 becomes higher than the reference voltage Vref2, the differential amplifier circuit 313 increases the current I2 flowing to the collector of the NPN transistor 315. As a result, the current I3 mirrored by the mirror circuit increases, and then a base current (I1-I3) of the NPN transistor 308 decreases. Because the base current of the NPN transistor 308 decreases, the output voltage Vdry of the common source amplifier circuit increases to decrease the output current Iout. In this manner, a negative feedback for controlling the output current is formed to control the output current lout to an overcurrent protection set current.
In the related-art voltage regulator as described above, however, the negative feedback for controlling the output voltage and the negative feedback for controlling the output current operate during the operation of the overcurrent protective circuit. Accordingly, when the gain of the negative feedback for controlling the output current is larger than the gain of the negative feedback for controlling the output voltage, the output voltage Vout is liable to oscillate, and hence a phase compensation circuit having a large capacitance is needed, and the chip area increases. Further, when the gain of the negative feedback for controlling the output current is smaller than the gain of the negative feedback for controlling the output voltage, the influence of the negative feedback for controlling the output voltage is large, and hence there is a problem in that the overcurrent protection for the output current lout hardly works.